It cannot be denied that the introduction of the internal combustion engine heralded a new era in transportation technology. The gasoline powered automobile has proliferated to the point where our nation's highways, and the streets of our large cities, are jammed with a bumper-to-bumper collection of automobiles signalling the daily rush hours.
There has always been a concern with efficient operation of internal combustion engines, but the ubiquitous automobile has focused concern on engines that burn both cleanly and efficiently. Improper combustion gives rise to byproducts that are damaging to the environment, and gasoline burning engines manufactured today are required to meet ever-tightening standards for combustion byproducts.
Of course, even in the early days of engine manufacture, it was thought that the single spark plug, poised at the top of the engine combustion chamber, caused uneven burning by virtue of its centrally located spark even in the presence of a stoichiometric mixture. Today's engines, restricted to leaner mixtures in the quest for emission reduction, need considerable assistance in achieving efficient combustion.
Design engineers have been restricted to similar engine configurations for some years, with gasoline burning engines having combustion chambers that are crowned by a cylinder head that must provide a mounting surface for at least intake and exhaust valves, as well as some means for igniting the fuel-air mixture. Many modern engines have multiple-valve configurations, further restricting the space available at the cylinder head for both ignition mechanisms, such as conventional spark plugs, and for sensors that can be employed to monitor combustion parameters.
It has been suggested that the uneven ignition provided by a single spark plug in the cylinder could be alleviated with ignition mechanisms that could be introduced into the combustion chamber through the cylinder head gasket. The head gasket also provides a likely spot for introduction of sensors that can permit real-time monitoring of combustion within the engine cylinders. Although the combustion process can certainly be regulated via an open-loop control system, such as the familiar method of establishing ignition timing with reference to top-dead-center of a reference cylinder, this technique is not easily adaptable to the changing conditions within the modern lean-burning engine.
Dynamic perception of the flame-front profile within the combustion chamber has been theorized as one way of permitting a wide range of controls to be exercised over engine operation. Strategically placed ion probes can detect the flame front, and even create a picture of flame-front propagation over time when used in conjunction with a microprocessor engine control system. Other combustion related measurements, along with sensor arrays designed to accomplish the indicated measurements, have also been suggested as workable techniques for real-time combustion monitoring. These sensor arrays have included pressure sensors, resistance-temperature devices (RTD's), knock sensors, and infrared sensors, as well as ion probes.
Implementation of these sensor systems is made difficult by the hostile environment of the engine itself. As is well-known, the combustion chamber must be thoroughly sealed, and this seal is generally accomplished through the use of a head gasket that fits between the cylinder head and the block of the engine. The head gasket is made from a variety of temperature resistant materials. If sensors are to be introduced through the space between the cylinder head and block, the sensors and associated wiring must be interposed in this space. One method for accomplishing this involves the use of a printed circuit board laminate with a substrate composed of a high temperature polymer, such as FR4. Layers of copper are bonded to one or both sides of the substrate, then selectively etched away to form circuit paths, as is well-known in the printed circuit board art. Multiple circuit layers can be formed by alternately bonding additional substrate and copper layers to an etched circuit board, again as is well-known. Since head gaskets seal by virtue of high compressive sealing stress, compressive loads at the elevated temperatures near the combustion chamber can cause damage to the circuit paths.
Accordingly, a need arises for a gasket enclosed sensor system that is durable enough to withstand the harsh combustion chamber environment while maintaining the integrity of sensor signal pathways. Such a system should also be relatively economical to manufacture and install.